1. Field of the Invention
The present invention relates to the coating of nylon films with vinylidene chloride copolymers.
2. Description of the Prior Art
It is known to use aqueous dispersions of vinylidene chloride copolymers containing 80 to 95%, by weight, of vinylidene chloride for coating purposes according to the disclosure in U.K. Pat. No. 959,455 to Farbwerke Hoechst AG, published June 3, 1964. This patent discloses that dispersions of copolymers having vinylidene chloride contents greater than about 92%, by weight, are preponderantly amorphous. However, such copolymers soon change after drying, in most cases within a few days, into a crystalline state and are, therefore, often referred to as "crystalline vinylidene chloride copolymers." The crystallinity index of crystalline vinylidene chloride copolymers, as measured by infrared spectroscopy, is generally above 1.15. It is also reported that dispersion of copolymers having less than 92%, by weight, of vinylidene chloride are also amorphous and that this amorphous structure may last for weeks or months after drying of the copolymer. The latter copolymers are often referred to as "amorphous vinylidene chloride copolymers." The crystallinity index of amorphous vinylidene copolymers, as measured by infrared spectroscopy, is generally 1.15 or less.
Coatings of the vinylidene chloride copolymers which remain in the amorphous form tend to exhibit a blocking effect. As used herein the term "blocking" refers to the well-known property of some films wherein difficulty is experienced when attempting to peel a layer of the film from a layer of a similar or different film. Although the blocking effect of amorphous vinylidene chloride copolymers may be overcome to a large extent by the addition of slip additives, such copolymers are not as impermeable to gases, e.g., oxygen, as are the crystalline copolymers.
In addition to the above, the amorphous copolymers tend to absorb volatile organic solvents. Such a propensity to absorb organic solvents is disadvantageous when the vinylidene chloride copolymer coating is laminated to other webs, e.g., polyolefin films. Organic solvents which are present in the adhesive, e.g., polyurethane adhesive, used to laminate the vinylidene chloride copolymer to the web, tend to become trapped in the vinylidene chloride copolymer coating. Such entrapment may lead to migration of the solvent into packages, e.g., packages of processed meats, made from the laminate, and to poor bonding of the vinylidene chloride copolymer to the webs to which it is laminated. Because of the disadvantages associated with vinylidene chloride copolymers which remain substantially amorphous, vinylidene chloride copolymers which become crystalline tend to be favored for coatings because of their resistance to absorption of organic solvents. However, crystalline vinylidene chloride copolymers do not readily adhere to nylon film.
In the manufacture of nylon film coated with crystalline vinylidene chloride copolymers, it is desirable to improve the adhesion of the copolymer to the nylon film through the use of an intermediary primer composition. One such primer is a polyurethane, as disclosed by D. G. James in U.S. Pat. No. 3,514,367, issued May 26, 1970. Other primers are disclosed in the aforementioned U.K. Pat. No. 959,455. A disadvantage of certain of these primers is that volatile organic solvents are required for application of the primer to the nylon film. In the workplace, aqueous systems would be preferred, as the hazard of flammability of organic solvents is not present. Notwithstanding any desire to use aqueous systems for coating nylon films, the tendency of nylon films to absorb moisture when in the presence of water and the resulting tendency of the nylon film to curl has inhibited the use of such systems.
Aqueous dispersions of vinylidene chloride copolymers which remain in the amorphous form, even at temperatures between ambient and the melting temperature of the vinylidene chloride copolymer, have been tested as a primer for nylon film, and the primer-coated film has then been further coated with a crystalline vinylidene chloride copolymer. However, such amorphous vinylidene chloride copolymer coatings, when brought into contact with moisture, turned hazy and gave a turbid appearance to the film. Such an appearance may be objectionable to consumers who wish to obtain a clear view of materials, e.g., food packaged in the vinylidene chloride copolymer coated nylon film.
It has now been found that amorphous vinylidene chloride copolymers which crystallize slowly under ambient temperatures and which crystallize relatively easily at elevated temperatures, e.g., 40.degree. C., may also be used as primers.